1. Field of the Invention
The present invention relates to an optical signal detector and an optical signal detecting method and particularly, to an optical signal detector which has two or more photodetectors and a function of calculating the difference between the outputs of two of these photodetectors to output a differential signal, and an optical signal detecting method for the optical signal detector.
2. Description of Related Art
In an optical signal detector having two or more photodetectors, when the difference between the output signals of two of these photodetectors is taken to create a differential signal, the relationship between the incident light and electrical output of the two photodetectors, that is, the photoelectric conversion efficiency is required to be relatively constant between the two photodetectors.
However, in order to make the photoelectric conversion efficiency uniform between the photodetectors, the photodetectors available are limited to only photodetectors which have relatively low photoelectric conversion efficiency and uniform characteristic. Therefore, the photoelectric conversion efficiency cannot be enhanced by the optical signal detector for producing the differential signal.
On the other hand, a photoelectric conversion element such as an avalanche photodiode having a high photoelectric conversion efficiency may be used to implement an optical signal detector having a high photoelectric conversion efficiency. However, the avalanche photodiode has a large dispersion in photoelectric conversion efficiency between elements, and thus it is unsuitable for producing the differential signal. Therefore, it has been difficult to implement a differential signal detector having a high photoelectric conversion efficiency.
The following publications are related to the present invention: Japanese Laid-open Patent Publication No. 61-158042discloses that DC offset between first photodetecting means and second photodetecting means is detected and then supplied to an input of an operational amplifier, and Japanese Laid-open Patent Publication No. 7-282458 discloses that the gain of the amplifier that the first photodetection output from a first photodetecting element is amplified is controlled and consequently, the balance between the first photodetection output and the second photodetection output from a second photodetecting element can be controlled to an optimum state.
An object of the present invention is to provide an optical signal detector and an optical signal detecting method which can produce a differential signal with no dispersion in differential detection and with high photoelectric conversion efficiency.
In order to attain the above object, according to a first aspect of the present invention, an optical signal detector which comprises a first photodetector, a second photodetector, differential output means for taking the difference between the outputs of said first and second photodetectors and outputting a differential signal, and control means which detect the difference in photoelectric conversion efficiency between said first and second photodetectors on the basis of the outputs of said first and second photodetectors and vary the photoelectric conversion efficiency of said second photodetector in accordance with said difference in the photoelectric conversion efficiency.
In order to attain the above object, according to a second aspect of the present invention, an optical signal detecting method for taking the difference between the outputs of a first photodetector and a second photodetector and outputting a differential signal, which comprises detecting the difference in photoelectric conversion efficiency between said first and second photodetectors on the basis of the outputs of said first and second photodetectors, and varying the photoelectric conversion efficiency of said second photodetector in accordance with said difference in the photoelectric conversion efficiency.
According to the present invention, even when photodetectors such as avalanche photodiodes which are difficult to be uniform in photoelectric conversion efficiency are used, the outputs of the two photodetectors needed to produce a differential signal are compared with each other to detect the difference in photoelectric conversion efficiency between the first photodetector and the second photodetector, and for example a bias voltage is applied to the second photodetector in accordance with the difference of the photoelectric conversion efficiency so that the two photodetectors apparently have the same photoelectric conversion efficient, thereby enhancing the detection precision of the differential signal. Accordingly, there can be implemented a differential signal detecting device using avalanche photodiodes having high photoelectric conversion efficiency or the like.